How to create few transparent, z-order independent QGraphicsItem?

[flart]

Is it same?

// circles 1,2,3,4 at image 
QGraphicsEllipseItem* item = new QGraphicsEllipseItem();
item->setBrush(QColor(255,255,255));
item->setPen(Qt::NoPen);
item->setOpacity(0.5);
myscene->addItem(item);

// circles 5,6 at image
QGraphicsEllipseItem* item = new QGraphicsEllipseItem();
item->setBrush(QColor(255,255,255,128));
item->setPen(Qt::NoPen);
myscene->addItem(item);

How create few z-order independent transparent object?

item1->setOpacity(0.5);
item2->setOpacity(0.5);

myscene->addItem(item1); myscene->addItem(item2); 
// vs
myscene->addItem(item2); myscene->addItem(item1); // different result, circles 1 2 vs 3 4 at image

Image:

[kenchan]

By default the stacking order is the draw order. You can set the zorder for the graphics itmes using the setZValue() method, whc\ich i what i think you want to do...

void QGraphicsItem::setZValue(qreal z)

From the docs here

void QGraphicsItem::setZValue(qreal z)
Sets the Z-value of the item to z. The Z value decides the stacking order of sibling (neighboring) items. A sibling item of high Z value will always be drawn on top of another sibling item with a lower Z value.
If you restore the Z value, the item's insertion order will decide its stacking order.
The Z-value does not affect the item's size in any way.
The default Z-value is 0.
See also zValue(), Sorting, stackBefore(), and ItemStacksBehindParent.

[flart]

@kenchan I knew that.

I want not stacking by z-order, I want simultaneous stacking.

It works something like that by default (if opacity of items is 0.5 )

output = Merge(Merge(background, 0.5 × item1), 0.5 × item2)

I want something like that

output = Merge(background, 0.5 × item1, 0.5 × item2)

aka simultaneous merge items to scene, so it not depends from z-order.
(example — I want ⅓ background, ⅓ item1 and ⅓ item2, Indifferently to items z-order)